The Localizer is part of the Instrument Landing System (ILS). The ILS is a ground based precision approach system that provides course and vertical guidance to landing aircraft. The Localizer is the component of the ILS that provides course guidance to the runway. The Localizer is located at the end of the runway. The Localizer equipment has 8, 14, or 20 antenna elements wired in an array. The number of antennas in the array depends on whether the airport has close or parallel runways, or if there are building obstructions near the runways. In these cases, the radiated signal pattern must be narrowed. Increasing the number of antennas in the array narrows the radiated signal pattern. All the proper signals have to be radiated from all the antennas to give a pilot the correct left-to-right indication on his instruments. A fault in the cable feeding an antenna will cause the Localizer system to shut down, and the pilot will lose the Localizer signal that could possibly result in an unsafe condition during final approach to landing.
The Localizer equipment generates a DC voltage that is present at all times in all the antenna lines in the antenna array. When the Localizer equipment is operating normally, a cable fault card in the equipment generates an audio tone and sends it back to the Localizer equipment shelter. When a cable fault occurs, the DC voltage on that particular antenna line goes to zero, and the cable fault card has group of circuits that detect this condition. When the DC voltage goes to zero on any of the antenna lines, the audio tone is inhibited and the Localizer monitor generates a cable fault alarm. However, the Localizer monitor does not identify which antenna in the array caused the fault.
Troubleshooting to find the fault requires the technician to measure voltages at a test point with a voltmeter to determine which antenna or feed line is has the fault which caused the Localizer system to shut down. The problem is compounded if the fault is intermittent. An intermittent fault can take hours or even days to troubleshoot. The technician has to be on the proper test point and actually see the voltage drop with a voltmeter to identify the antenna. The technician must start at one end of the antenna array and check every connector. In a 20-element antenna array there are 80 connectors. There have been instances where an antenna array was completely recabled, and upon the completion, the problem was still present.